The present invention relates to a method for coating workpieces by arc evaporation whereby the workpieces are remote from an area in direct view of the evaporation cathode. Further, the present invention relates to a vacuum coating apparatus for coating workpieces with at least one evaporation cathode arranged within a process chamber which may be evacuated and which contains at least one anode and further comprising a carrier arrangement for workpieces outside of direct view of the cathode.
The present invention is directed to the general technique of evaporating electrically conductive materials using cathode spots of an arc discharge.
This technique of evaporation is particularly suited for plasma aided coating, because the cathode spots do not substantially emit cathode material as a neutral vapor, as do other sources for vacuum coating, but rather predominantly emit ions The ions form, together with the electrons emitted as well, a plasma with a high density. Such a plasma allows coating with a high efficiency. Additionally, most favorable coating properties, such as high density and strength, low roughness, etc. are realizable by such plasma aided coating. This is valid for coatings which are formed by depositing metal vapor of the cathode material alone, as well as and especially for coatings formed from a compound of the metal with a reactive gas which is admitted into the remaining gas atmosphere within the process chamber. With the use of plasma aided vacuum coating techniques, the variety of compounds which may be reactively realized by addition of components from different sources during deposition on a substrate has been considerably extended beyond the well-known deposition of oxides.
In the present description, cathodes which are submitted to an electrical arc discharge, so that their material is heated by this arc to such an extent that it evaporates, are referred to as evaporation cathodes. Such a cathode is an electrically negatively supplied electrode emitting not only electrons for sustaining the arc, but also supplies the arc with ions, i.e., ionized atoms evaporated from the cathode. Of course, arc discharge with a completely cold cathode needs ignition in addition to connection to a d.c. supply, as is known in the prior art later cited. Because evaporation occurs at the cathode surface only at points, i.e. at the roots of the arc discharge which stochastically move along the cathode surface, and such points leave the impression of arcs to an observer, this evaporation method is often referred to as the arc evaporation technique.
For well-known arc evaporation techniques using flat evaporation cathodes, the directional distribution of the emitted ions follows substantially the cosine-law. This means that the stream of evaporated material which occurs in a direction tilted by an angle .alpha. and with respect to a perpendicular to the cathode surface, will be smaller by a factor of cos.alpha. compared with the emission in said perpendicular direction. From the point of view of economic coating it is therefore advisable to arrange the substrates to be coated near a perpendicular axis on the middle of the evaporation cathode. When evaporating from cathode spots, this was additionally advisable up to now, because a substantial part of the material from the surface of the cathode is ejected in the form of liquid droplets, with a directional distribution which substantially differs from the directional distribution of the ions. Most, and especially the larger droplets, leave the cathode with a significant tangential component of trajectory. This is due to the mechanism of the arc discharge, the spots of which are freely movable on the cathode surface. Because the elementary mechanism is an evaporation at locally extremely overheated areas in the neighborhood of the arc spot, there is always a liquid phase of cathode material from which a part is ejected as droplets due to the vapor pressure of vapor generated at the area of molten material.
The droplets may solidify on their way to the workpieces or when they hit the substrates or workpieces an thus cause different disadvantageous qualities of the coatings, e.g. higher roughness of the coating surface, contamination of the coating, pronounced tendency of corrosion after grains resulting from droplets are broken off of the substrates etc. The least droplets are encountered on workpieces which are arranged in the neighborhood of the perpendicular axis, i.e., there where the coating efficiency is the largest.
Particularly, the present invention is directed to said phenomenon of droplets occurrence.
Known arrangements for vacuum coating using evaporation from cathode spots have attempted to exploit the different emission characteristics of vapor and droplets to generate coatings with the smallest possible number of droplet-caused deficiencies. For this purpose the substrates are arranged close to each other within a small area (relative to the diameter of the cathode) and around the axis of the cathode, with the largest possible distance from the cathode. The required large distance of the workpieces from the cathode on the one hand and the small number of workpieces which may be arranged near the axis, does not allow commercial production. Therefore, when using so-called arc evaporation, compromises were taken e.g. use of smaller distances or larger areas for the workpieces, which resulted in a substantial degradation of the coating quality due to the high proportion of droplets in the coatings. Therefore, different efforts have been directed towards reducing the number of droplets.
Some approaches to avoid the above problems separate the desired ions of cathode material from the undesired droplets by plasma optical techniques. Principally, these approaches always reside in leading the ions into an area which is not in direct view of the cathode. There the workpieces may be arranged and will not be hit by droplets, because the mass to charge ratio of such droplets is different from that of single ions by several decades.
One approach (known e.g. from DE-PS 32 34 100) provides for a rotational symmetrical carrier for workpieces along the axis of a container. Around said carrier there is arranged an evaporator of larger radius and of ring-like configuration. Its evaporating surface is not directed towards the substrates, but is directed outwards. The cylindrical wall of the container acts thereby as an ion-reflecting element.
Here the area which may be used for arranging workpieces to be coated simultaneously is restricted to the limited area adjacent the axis of the container.
Another approach, known from FIG. 2 of U.S. Pat. No. 4,452,686, exclusively utilizes the ions emitted along the axis of a rotational symmetrical container and leads these ions by reflection at the wall of the container around an obstacle which collects the droplets into a droplet-free area, wherein the workpieces are arranged.
The area which may be utilized for arranging workpieces to be simultaneously coated is restricted to that part of the cross-sectional area of the container which is also the umbra of the obstacle for keeping off the droplets. Although this area will increase with decreasing diameter of the cathode, only a few workpieces may be coated simultaneously. Thus, at least in an area adjacent such obstacles the overall cross-sectional area of the container may not be exploited equally well for arranging workpieces to be coated.
From the article "Transport of plasma streams in a curvilinear plasma-optics system" by I. I. Aksenov et al, Soviet Journal Plasma Physics, 4(4), July-August 1978, it is know to lead the evaporated ions along a deflecting tube towards a workpiece, so that, again, a direct path between workpiece an cathode is avoided. The workpiece is arranged at an open end of the deflecting tube and line of sight contact is prevented by a bend in the tube. The area which is exploitable here for workpieces to be simultaneously coated, is also relatively small.